More than two-thirds of all the superalloys produced are consumed by the aerospace and automotive industries. The remaining portion of superalloy consumption is used by the chemical, medical, and structural industries in applications requiring high temperature properties and/or exceptional corrosion resistance. The ability to retain high mechanical and chemical properties at elevated temperatures make superalloys ideal for use in both rotating and stationary components in the IC engine of an automobile or in the hot end of a jet engine. These materials as well as structural ceramics and hardened steels pose formidable challenges for cutting tool materials during machining; hence they are referred to as difficult-to-cut.
Conventionally in the machining industry, generating a component from raw goods includes a casting or forging process, rough machining, heat treatment to a desired hardness, and then finished-machining through a grinding process. Grinding has several disadvantages, which include high specific energy consumption and low material removal rates.
Hard turning is a machining process whereby “hard” workpieces, for example, such as steel, are shaped by moving a cutting tool against the workpiece. The cutting tool may be stationary or rotary. High cutting temperatures are generated during hard turning. The generated temperatures cause thermal softening of the workpiece material in the cutting zone leading to reduced cutting forces. The reduction of generated force is desirable. However, excessive temperatures generate thermal damage on the machined surface as well as soften the cutting edge leading to plastic deformation. The high specific forces and temperatures affect the modes of tool wear in hard turning. The generated tool wear affects the integrity of the generated surface and therefore controlling it is a major challenge. The adverse effect of heat on the tool tip can be reduced by using cutting fluid or by continuously supplying a fresh cutting edge, as is the case in rotary cutting tools.
Rotary tool cutting involves a tool in the form of a disk that rotates about its axis. Different types of rotary tools have been developed, all with similar functional characteristics, however few are commercially available. Rotary tools can be classified as either driven or self-propelled. The former is provided rotational motion by an external source while the latter is rotated by the chip flow over the rake face of the tool.
Rotary cutters are known to reduce the heat generated during turning operations and can result in the rotary cutter itself enjoying a longer life span compared to non-rotating cutters. Non-rotating cutters induce higher cutting forces, and generate higher heat. Coolants can be used with non-rotating cutters to combat the heat, but such coolants are generally environmentally hazardous.
In prior art rotary cutters, the inserts, which may be mounted at the tip of the tool holders, may be externally driven or self-propelled. Self-propelled cutters include a cutting insert that is rotated by the rotating work piece. As self-propelled rotary cutting tools do not require an external energy source to be rotated, the result is that self-propelled rotary cutting tools have reduced energy consumption during metal processing operations as compared to externally driven rotary cutting tools. There are few prior art self-propelled rotary tools that are commercially available.
Typically, cutting tool material hardness of at least three times harder than the work material is recommended. Cutting materials such as ceramic and polycrystalline cubic boron nitride (PCBN) are recommended for turning hardened steel because of their ability to sustain the high temperature generated during the metal removal process. Hard turning with ceramic cutting tools has been a time proven manufacturing process that may replace some grinding applications.
The basic difference between rotary cutting and conventional cutting is the movement of the cutting edge in addition to the main cutting and feed motions. Self-propelled rotary tools (SPRT) employ round inserts that rotate continuously about their central axis as a result of the driving motion impacted by the cutting force, thus minimizing the effect of thermal energy along the entire edge and preventing excessive heating of a particular portion of the cutting insert. Major benefits provided by rotary cutting tools include several hundred-fold increase in tool life, lower cutting temperatures, higher metal removal rates, generation of fine surface finishes due to the circular cutting edge, and improved machinability of difficult-to-cut materials such as nickel and titanium based alloys. Extremely low rate of flank wear can be obtained when machining superalloys, especially titanium alloys, even at higher speed conditions with negligible or no effect on the machined surfaces.
Machinists generally have interest in utilizing self-propelled rotary tools (“SPRTs”) due to the economical benefits of machining with SPRTs. However, prior art SPRTs for hard-turning are not widely utilized by machinists in smaller machining facilities due to the high costs required to purchase such SPRTs and to utilize such tools. Two leading manufacturers of rotary tools, namely Rotary Technologies™ and Mitsubishi™, require specifically designed components and inserts for their production of prior art SPRTs for hard turning applications.
Machining by turning basically generates cylindrical forms with a single point tool. The cutting tool remains stationary while the workpiece rotates. This process is one of the most straightforward metal cutting methods with relatively uncomplicated definitions. However, being one of the most widely used machining methods, turning has become a highly optimized process. To maintain high efficiency requires the thorough appraisal of the various factors involved in applications.
Orthogonal and oblique cutting are the two most fundamental and conventional prior art machining types. The straight cutting edge on the tool used in orthogonal cutting 20 is positioned normal to the cutting velocity direction, as shown in FIG. 1. The depth of cut that the cutting edge engages into the workpiece is referred to as the chip thickness ‘t’.
Conventional oblique cutting is similar to conventional orthogonal cutting with the exception of the straight cutting edge on the tool used in oblique cutting 22 being inclined with an acute angle from the cutting velocity direction, as shown in FIG. 2. This acute angle is referred to as the inclination angle, V′ and similar to conventional orthogonal cutting, the tool cutting edge is engaged into the workpiece at a depth of cut ‘t’. Conventional oblique cutting and orthogonal cutting are disclosed in M. C. Shaw, P. A. Smith and N. H. Cook, 1952, “The Rotary Cutting Tool,” Transactions of the ASME, pp. 1065-1076.
For orthogonal cutting, there are two basic cutting surfaces of the workpiece: the work surface being the surface of the workpiece to be removed by the machining process; and the machined surface being the surface produced after the cutting tool passes. One additional surface may be considered for many practical machining operations: the transient surface: the surface generated during cutting by the major cutting edge. This surface is always located between the work surface and machined surface, as disclosed in G. R. Nagpal, 1999, “Machine Tool Engineering,” Khanna Publishers. This last surface distinguishes orthogonal cutting from other machining processes, for example, in accordance with the shaping, planning, and broaching, where the cutting edge is perpendicular to the cutting speed. The machined surface is generated from the tool nose and minor cutting edge, both of which directly affect the integrity of the machined surface including residual stresses and finish quality.
Depending on the geometry of the cutting tool, various mechanics, thermal reactions, and tool wear conditions will arise during cutting. There is a wide array of cutting tools for various cutting methods, such as turning, milling, drilling, broaching, and reaming. However, there is a distinct lack of information regarding cutting tool geometry and its influence on the outcomes of machining operation. In the past, computers were not available to calculate parameters of such geometry. This made the task of reproducing proper tool geometries with manual machines difficult. Recent improvements in the machining industry have created more focus on tool design, primarily including tool materials and geometry, as a means of improving the performance of cutting tools.
In particular, the cutting tool geometry can be important because it may directly affect: (i) chip control, as tool geometry defines the direction of chip flow and the direction is important to control chip breakage and evacuation; (ii) productivity of machining, as the cutting feed per revolution is considered one of the major resources in increasing productivity because feed can be significantly increased by adjusting to tool cutting edge angle (for example, milling utilizes this parameter to a large extent where it is found that increasing the lead angle to 45° allows the feed rate to be increased approximately 1.4-fold and as a result, a wiper insert is required to reduce feed marks left on the machined surface due to the increased feed rates); (iii) tool life, as this geometry defines the magnitude and direction of the cutting force and its components and these include the sliding velocity at the tool-chip interface, the distribution of thermal energy released in machining, the temperature distribution in the cutting edge, etc., all of which affect tool life; (iv) the direction and magnitude of the cutting force and thus its components as four components of importance in the cutting tool geometry include the rake angle, the tool cutting edge angle, the tool minor cutting edge angle, and the inclination angle, all of which define the magnitudes of the orthogonal components of the cutting force; (v) quality, including surface integrity and residual stress of machining, as the comparison between tool geometry and the theoretical topography of the machined surface is common knowledge, and cutting geometry influences the machining residual stress which is realized when one recalls that the geometry defines to a great extent the state of stress in the deformation zone, for example, such as around the tool.
The geometry of prior art cutting tools, in particular the tool-in-hand tool geometry, has followed two basic standards: (a) the American National Standard B94.50-1975 “Basic Nomenclature and Definitions for Single-Point Cutting Tools 1”, reaffirmed date 2003; and (b) ISO 3002/1 “Basic quantities in cutting and grinding—Part 1: Geometry of the active part of cutting tools—General terms, reference systems, tool and working angles, chip breakers”, second edition 1982 Aug. 1. These standards have however failed to remain current and do not account for the significant changes in the machining industries and for the advances in metal cutting theory and practice.
Prior art SPRTs include tools created by Rotary Technologies™ and Mitsubishi Materials™ for hard turning processes. Each of these companies manufacture tools that are designed to be comprised of components that are proprietary to each company, such components including inserts, bearing assemblies, seals, hardware, and other components. These tools cannot be maintained in operation and serviced in an economical fashion. As the tools are comprised of proprietary components, both Rotary Technologies™ and Mitsubishi Materials™ can demand higher costs for the sale of their tools and the replacement of components of such tools. The prior art tools manufactured by each company further comprise complex assemblies, which further complicate the serviceability and maintenance of such tools.
For example, the Rotary Technologies™ SPRT for hard turning utilizes simple disk inserts with a cutting edge diameter of 25.4 mm. These inserts are not standard ISO inserts and therefore are not readily available from other tool manufacturing companies. The inside diameter of the insert causes the geometry of the tool to be proprietary to Rotary Technologies™ SPRT.
As another example, Mitsubishi Materials™ uses a 12.7 mm insert, but the geometry on the base of the insert (opposite the cutting edge) makes the tool unique compared to the ISO inserts commercially available. The use of large diameter inserts can result in improved surface quality, however they can simultaneously generate larger thrust forces during machining which can lead to increased tool chatter if the machine tool is not sufficiently rigid.
Further examples of prior art cutting tools are disclosed in the following patents and patent applications: U.S. Pat. No. 4,065,223 (Nelson); U.S. Pat. No. 4,640,159 (Stojanovski); U.S. Pat. No. 3,777,341 (Faber); Canadian Patent No. 1,335,152 (Massa); Canadian Patent No. 1,002,307 (Munro); U.S. Patent Application Publication No. 2001/0013995 (Hecht); Canadian Patent No. 832,722 (Cashman); EPO Patent No. GB2053765 (Kemmer); EPO Patent No. GB2352415A (Keith); U.S. Pat. No. 7,153,069 (Van Horssen); U.S. Patent Application Publication No. 2005/0047885 (Hyatt); U.S. Pat. No. 3,329,065 (Vaughn); U.S. Pat. No. 4,515,047 (Komanduri); U.S. Pat. No. 5,014,581 (Komanduri); U.S. Pat. No. 5,478,175 (Kraemer); U.S. Pat. No. 6,073,524 (Weiss); U.S. Pat. No. 6,135,680 (Szuba); U.S. Pat. No. 7,156,006 (Hyatt); and U.S. Pat. No. 7,325,471 (Massa).